The present invention relates to emission control systems and more particularly, an emission control system for adjusting the air-to-fuel ratio of an internal combustion engine based upon a measurement of the exhaust gas temperature of the engine.
There are many new technologies being developed and existing technologies being refined to meet ever more stringent automotive exhaust emission standards. The two general areas of development for reducing automotive exhaust emissions are: (1) reducing engine generated exhaust emissions and (2) optimizing after-treatment of engine generated exhaust emissions.
Automotive tail pipe emissions are conventionally minimized by closed loop control of engine air and fuel by way of feedback from an exhaust gas oxygen (EGO) sensor mounted in the engine exhaust path. The EGO sensor output signal regulates the engine air-to-fuel (A/F) ratio by adjusting the engine fuel injection period for each cylinder event. A system of one or more three-way catalytic converters for after treatment of exhaust gases in combination with closed loop A/F ratio control provides a substantial reduction of tail pipe emissions.
However, neither the EGO sensor or the catalytic converter are immediately effective when a cold engine is first started. Catalytic converters must attain a critical temperature (i.e. the light-off temperature) before they are operative. The period of time prior to catalytic converter light-off is known as the cold start period and generally lasts about 30 seconds. Similarly, EGO sensors are electrically heated and require 10-15 seconds before the EGO sensor output can be used for closed loop control of the A/F ratio. Because EGO sensors require a warm-up time, and because a 10-15 second wait between ignition activation and the start of cranking is generally thought to be unacceptable to drivers, the control of automotive engines is preset to operate open loop, without benefit of EGO sensor feedback, for the first 10-15 seconds of operation. Thus the fuel injector periods are preset to achieve a predetermined A/F ratio based on assumed engine and fuel parameters during the cold start period.
The actual A/F ratio in an engine combustion chamber is a function of the volatility of the fuel. Fuel having a lower volatility results in a higher A/F ratio within the combustion chamber than higher volatility fuel. The volatility of fuel is characterized by a parameter referred to as the driveability index (DI) (see FIG. 1). The higher the driveability index, the lower is the volatility of the fuel. The DI of manufactured gasoline varies with grade and season, the normal range being from 850 to 1300. Further, the DI of the fuel delivered to an engine may vary due to evaporation. Thus, the DI of the fuel actually supplied to an engine cannot be accurately predetermined.
During warm engine operation, the output signal from the EGO sensor is effective to compensate for the variable DI of the fuel. However, as shown in FIG. 2, during the cold start period of internal combustion engine operation, when the EGO sensor is inactive and the regulation of A/F ratio is open loop, fuel having a high DI (curve A) causes the A/F ratio of fuel in the engine combustion chamber to shift in the lean direction compared to standard DI fuel (curve B), resulting in unacceptable vehicle driveability, i.e. hard starting, rough idle, poor throttle response and stalling. In order to compensate for the lean shift of the A/F ratio during the cold start period caused by high DI fuel, the open loop A/F ratio of automotive engines is generally preset to be richer than for standard fuel (i.e. DI=1100) to provide acceptable vehicle driveability in the event that the fuel supply has a high DI (i.e. DI=1275). The result is that when standard driveability fuel is in use, the A/F ratio is too rich, undesirably increasing hydrocarbon (HC) emissions. Since it is likely that the DI of the fuel is standard, and since up to 80% of automotive HC tail pipe emissions under federal test procedure FTP 75 occur during the cold start period, the increase in HC emissions due to unnecessarily compensating for the unlikely presence of high DI fuel is significant.
If the DI of the fuel could be quickly determined, it would not be necessary to program the A/F ratio to be overly rich. Experimental data demonstrates that the temperature of the exhaust gas of an internal combustion engine is a function of the A/F ratio (see FIG. 3). Furthermore, computer models currently in use in existing engine control systems can predict the temperature of the exhaust gas with acceptable accuracy when provided with information on engine speed, engine load, A/F ratio and engine timing. Consequently, the presence of high DI gasoline is capable of being detected by measuring the temperature of the exhaust gas of an internal combustion engine and comparing the measured exhaust gas temperature with the temperature that would be produced by standard DI gasoline as predicted by the exhaust gas temperature prediction model. FIG. 4 shows experimental data that demonstrates a measurable difference in exhaust gas temperature at the beginning of the cold start period when high DI fuel (curve A) is used, compared to the exhaust gas temperature resulting from using standard fuel (curve B).
The present invention, by initially setting the engine A/F ratio for standard DI fuel, optimizes the operation of the engine by providing acceptable vehicle driveability with reduced HC emission during the cold start period, compared to the conventional method of initially enriching the A/F ratio on the chance that the fuel may have a high DI. The present invention uses an empirically derived computer model to provide a prediction of the exhaust gas temperature that results from using standard DI fuel. As the engine warms up, the actual exhaust gas temperature is measured with a fast response time exhaust gas temperature sensor and compared with the predicted exhaust gas temperature. If the actual exhaust gas temperature is higher than the temperature predicted by the computer model, high DI fuel is indicated. Accordingly, upon detecting the high DI fuel, the A/F ratio is made richer in proportion to the temperature difference between the predicted and actual values of the exhaust gas temperature.